This invention relates to solid state optical waveguide structures and to processes for fabricating them.
Optical waveguide structures based on silicon dioxide are used to prepare a variety of integrated optics devices. A typical waveguide structure includes a silicon substrate having three layers of silicon dioxide, each doped to produce a desired index of refraction and reflow properties. The first layer on the silicon substrate, called the lower cladding layer, typically comprises 10-15 microns of undoped silicon dioxide. The second layer, called the core layer, may comprise silicon dioxide doped with either phosphorus or germanium to increase its index of refraction relative to that of the lower cladding layer. The third, or upper layer of silicon dioxide, overlies the core layer and has an index of refraction close to that of the lower cladding layer and a thickness also in the range of 10-15 microns.
In many applications a single mode waveguide is desirable. This characteristic may be achieved by appropriate selection of the cross-section geometry and the index of refraction of the core layer. Typically, using a layered silicon dioxide structure, as described above, the cross-section of the core layer may be about 5 microns high and 7 microns wide. The preparation of the core layer of a single mode waveguide may involve deposition of a planar layer of core material on a silicon dioxide lower cladding layer. The core layer is then patterned and etched to form the desired geometry. The waveguide structure is completed by deposition of the upper cladding layer. The index of refraction of the core layer suitably exceeds that of the lower and upper cladding layers by about 0.01. This results in an optical mode that matches modes of most single-mode optical fibers and, at the same time, allows for the introduction of curved waveguide sections as may be needed in some device structures. Such curved sections must exhibit low propagation losses even with small radii of curvature (10-15 mm).
Two basic methods of fabricating such optical waveguide structures are typically employed. One is called flame hydrolysis in which sources of silicon dioxide and germanium dioxide, the latter used as a dopant, are injected into a hot torch flame. Fine particles of doped silica are formed in the flame of the torch and deposited on a cooler substrate. The flame may be rastered over a wafer in order to achieve uniform coverage. After deposition the core layer is annealed, at high temperatures, to form optical quality glass. All three layers may be formed, sequentially, in this manner. Since a flame is rastered over the wafer, very high precision motion is needed in order to obtain glass layers with uniform thickness and doping levels. These disadvantages become of particular concern when scaling up to process large wafer sizes and the process is significantly different from standard processes developed for silicon device fabrication.
The second method is based on chemical vapor deposition (CVD), typically implemented in conjunction with thermal oxidation. In this method, the lower cladding layer is formed either by high- pressure thermal oxidation of silicon, a process well established in semiconductor device processing and known as HIPOX, or by CVD. The core layer is formed by CVD of phosphorus doped silicon dioxide. In order to achieve the desired refractive index step between the core and cladding layers, it may be necessary to add as much as 8% of phosphorus, by weight, to the silicon dioxide. Once the core layer is formed, it is subjected to thermal anneal which produces optically transparent glass. Material prepared by this CVD process is known as P-Glass. The upper cladding layer is typically formed by introducing boron and phosphorus into a silicon dioxide CVD process to control the index of refraction and, at the same time, to make it possible to reflow the top cladding layer at a relatively low temperature.
Typically, a CVD method used to prepare optical waveguides relies on thermal decomposition of source gases. Low pressure in the deposition chamber minimizes gas phase reactions and assures thermal decomposition at the surface of the wafer. The core layer is deposited by thermal decomposition of silane (SiH4) and nitrous oxide (NO2), at a temperature of 450-500xc2x0 C. in the presence of phosphine (PH3) as a phosphorous source. The relatively low deposition temperature allows for high phosphorus concentration needed to form the effective core layer. However, boron cannot be introduced in this process. The preparation of the top cladding layer requires doping with boron and phosphorous, as mentioned above. Boron lowers the reflow temperature of the silicon dioxide and lowers the index of refraction. Co-doping with boron and phosphorous is used to simultaneously control the reflow temperature and the index of refraction. A convenient process developed for the preparation of boron and phosphorous doped silicon dioxide is CVD deposition from tetraethyloxysilane (Si(OC2H5)4), also known as TEOS. Standard CVD deposition using TEOS requires a higher temperature, suitably 750-780xc2x0 C. with trimethylborate (TMB) and trimethylphosphate (TMP) used as boron and phosphorous sources. The high deposition temperature needed in TEOS CVD does not permit incorporation of phosphorus at the concentration needed in the core layer. Thus CVD processes using different process parameters and source materials are required to form the core and upper cladding layers which is disadvantageous in terms of process control and time as well as economically. The lower cladding layer may be formed from undoped, thermally grown oxide. A layer of thermal oxide about 10 to 15 micron thick is typically needed in order to place the waveguide far enough above the silicon wafer to eliminate substrate-related losses. Because of the large thickness, this oxide layer is typically grown under conditions of high pressure (e.g. in excess of 10 atmospheres) and high temperatures (e.g. over 900xc2x0 C.) for times longer than 100 hours.
In one aspect, the invention provides a method of fabricating an optical waveguide structure, comprising: forming on a silicon substrate an optical waveguide core between lower and upper cladding layers of silicon oxide, each of said core and said upper cladding layers formed by plasma enhanced chemical vapor deposition from silicon, oxygen and dopant containing atmospheres such that said core has a higher refractive index than the refractive indices of each of the lower and upper cladding layers sufficient to enable waveguiding of optical signals introduced into said core. Preferably, the lower cladding layer is formed by plasma enhanced chemical vapor deposition from silicon, oxygen and a dopant containing atmosphere, suitably containing boron and phosphorous dopant sources. Suitably, the lower and upper cladding layers each has a thickness of at least 15 microns and the core a thickness of about 5 microns. Typically, the core is formed by patterning a deposited layer and to enhance conforming the upper cladding layer to the patterned core, the upper cladding layer is preferably formed by a succession of alternating deposition and annealing steps to build up a final layer of desired thickness. The lower cladding layer may alternatively be formed by thermal oxidation of the silicon substrate.
In a preferred embodiment, a method of fabricating on a silicon substrate, a waveguide structure comprising an optical waveguide core between lower and upper cladding layers of silicon oxide, comprises: forming said lower cladding layer, an intermediate layer and said upper cladding layer by plasma enhanced chemical vapor deposition from silicon, oxygen and dopant containing atmospheres. The atmosphere for depositing each of the lower and upper cladding layers includes both boron and phosphorous dopant sources, and the atmosphere for depositing said intermediate layer includes a phosphorous dopant source such that said deposited intermediate layer has a refractive index about 0.01-0.014 higher than the refractive index of said lower cladding layer and the refractive index of said upper cladding layer. The intermediate layer is patterned to define said waveguide core with a desired configuration prior to forming the upper cladding layer which preferably is formed by a succession of alternating deposition and annealing steps to conformally flow said upper cladding layer around said waveguide core, to convert said waveguide core to optical quality glass, and to anneal the lower cladding layer. The deposited lower cladding layer provides optical isolation of said waveguide core from said silicon substrate and said upper cladding layer optically confines within the waveguide structure, optical signals introduced into said waveguide core. Preferably, said lower and upper cladding layers each has a thickness of at least 10 microns and said intermediate layer has a thickness of about 5 microns.
In one embodiment, a method according to the invention may be employed to fabricate an optical waveguide structure in which the lower and upper cladding layers each comprises annealed silicon dioxide doped with about 4% boron and about 2.5-5% phosphorous; and said intermediate layer comprises annealed silicon dioxide doped with about 8% phosphorous. However, advantageously, the upper and lower cladding layers may comprise annealed silicon dioxide doped with boron in the approximate range 9% to 11% and with phosphorous in the approximate range 2.5% to 3.5%, to enable single mode waveguide propagation of optical signals introduced into the core which is doped with about 8% phosphorous, such that the thermal coefficients of expansion of the upper and lower cladding layers match that of the silicon substrate and stress in the core is reduced close to zero or with low residual tensile stress to enable birefringence induced optical signal phase shift between TE and TM modes of propagation of an optical signal in said core layer not exceeding about 0.1 and preferably below about 06 nm. Such an optical waveguide structure is described in greater detail in copending application Ser. No. 10/047,779, filed on the same date as this application by Henryk Temkin and Rudolf F. Kazarinov, entitled Optical Waveguide Structures, also assigned to Applied WDM, Inc., the disclosure of which is hereby incorporated by reference in the present application.